We report experiments investigating a resonant sum-difference frequency mixing scheme in krypton producing coherent radiation at 123.6nm (i.e. in the vacuum ultraviolet (VUV)). Owing to the resonant nature of this scheme, electromagnetically induced transparency (EIT) plays a crucial role in the generation of this coherent radiation, resulting in a greatly enhanced conversion efficiency. This is the first time a resonant sum-difference mixing scheme involving EIT, that generates VUV radiation, has been investigated experimentally. Our results are in broad agreement with theoretical predictions. Measurements are presented of the dependence of VUV generation on laser detuning (in the range ± cm−1) and laser powers. In particular the dependence on the coupling field strength has been studied in the range of coupling field Rabi frequencies Ωc = 0.05−3 cm−1. We estimate an enhancement in the conversion efficiency, compared with the non-resonant case, of greater than 102.
We report experiments that investigate the generation of coherent VUV radiation at 123.6 nm via a resonant sum-difference frequency mixing scheme in Kr. One of the mixing fields at 212.55 nm was in two-photon resonance with the transition between the
4p61S ground state and the
4p55p[0,21] excited state $(94093.7 {\mathrm{cm}}^{\ensuremath{-}1}).$ The second field (the coupling field) at 759 nm was in single-photon resonance with the transition between the
4p55p[0,21] state and the
4p55s[1,21] state at $80917.6 {\mathrm{cm}}^{\ensuremath{-}1}.$ The presence of the coupling field leads to electromagnetically induced transparency (EIT) at the wavelength of the generated field, 123.6 nm. This is predicted to enhance the four-wave-mixing efficiency by a large factor. We have studied the dependence of the four-wave mixing process on the detuning and strength of the coupling field. The efficiency for four-wave mixing was found to be enhanced by a significant factor (>5) by the EIT effect when the resonant coupling field strength exceeded about half the Doppler width $(0.1 {\mathrm{cm}}^{\ensuremath{-}1}).$ A calculation for monochromatic fields and a uniform slab of gas provided qualitative agreement with the results of experiment. The relative conversion efficiency for this resonant mixing scheme is found to be
104 times greater than that for a similar scheme where the coupling laser is tuned far $(1270 {\mathrm{cm}}^{\ensuremath{-}1})$ off resonance. A substantial part of this resonant enhancement can thus be attributed to the EIT effect.